1. Field of the Invention
The present invention relates to an auto exposure control circuit, and more particularly, to a dual exposure control circuit and associated method.
2. Description of the Prior Art
A conventional automatic exposure control loop applied in an imaging system or a camera system comprises a sensor, an auto exposure engine and an image signal processor (ISP), wherein the sensor comprises a photoelectrical device for transforming the received photon into an electrical signal, an analog-to-digital converter for transforming the electrical signal into a digital signal, and a digital/analog amplifier for adjusting a magnitude of the digital signal. Traditionally, the photoelectrical device is with an electrical shutter in a CMOS image sensor for controlling the exposure time of each frame while the gain of the amplifier is well known as ISO in an imaging system. The AE engine comprises a light metering unit for measuring a brightness level of the received image, a compensator for computing an exposure time or the gain for the up-coming frames and a decision circuit for issuing commands of pixel exposure time and amplification gains.
The exposure time and the gain are the most important factors in an imaging system for adjusting the luminance of received images. How to adjust these two factors harmoniously and simultaneously is always an issue, however; greater exposure time might have a higher Signal-to-Noise Ratio (SNR) but can easily cause motion blur, whereas greater gain might have greater noise which is always unwanted. Adjusting the exposure time is usually the first choice to reach the target brightness for the received image and obtain a better SNR, but the concerned exposure time is limited by frame rate and cannot be increased unlimitedly. Likewise, the gain of pixel array cannot be increased unlimitedly due to inherent limitations of technology.
The arrangement of the pixel arrays is another important factor for the imaging system. A Bayer pattern is the most common way to arrange the Color Filter Array (CFA) in an imaging system for better luminance of frame. Recently, in order to obtain more energy for the pixel array, a white pixel has been introduced into pixel arrays to enhance sensitivity of the image sensor due to the high transmittance of white pixels whose spectral responsivity is almost twice that of a green pixel. In a single exposure control circuit, the image usually suffers from over-exposure of the white pixel due to this higher light sensitivity. When over-exposure of white pixels occurs in a single exposure control circuit, a prior art method compensates the over-exposed white pixel by using the red/blue/green (RGB) pixel around the white pixel. This method still generates an unpredictable distortion in the image, however.